WASHINGTON, Sept 1 (Reuters) - U.S. military forces carried out an operation on Monday against al Shabaab militants in Somalia, a U.S. Department of Defense spokesman said.

"We are assessing the results of the operation and will provide additional information as and when appropriate," Pentagon spokesman Rear Admiral John Kirby said in a statement.

No further details about the operation in the African country were immediately available.

Al Shabaab is an Islamist group affiliated with al Qaeda that wants to impose its own strict version of Islam in Somalia. It ruled most of the southern region of Somalia from 2006 until 2011, when African peacekeeping troops marched into the capital, Mogadishu.

African and Somali forces have regained several towns this year, but rebels still hold other centers and tracts of countryside. (Reporting by Ian Simpson, editing by G Crosse)